The closing sequence of a set of double doors is often regulated in the prior art by a door coordinating mechanism. Such a device dictates that a specified one of the doors always closes first followed by the closing of the other door. The first door to close is often termed the "inactive" or "leading" door. The other (or last) door to close is appropriately termed the "active" or "trailing" door.
The prior art discloses automatic locking mechanisms particularly designed for coordinated double doors. For example, U.S. Pat. No. 3,578,369 shows a self-operating extension flush bolt or locking mechanism which is applied to the inactive door of a pair of hinged doors. When the inactive door is in a closed position and the active door is thereafter closed, the active door engages a cam projecting from the inactive door. This engagement action causes a latch bolt to project from a horizontal edge of the door into a strike plate located either in the door header or the door sill.
This patented locking mechanism requires considerable force to operate the bolt, which makes it necessary to use a heavy duty door closer for the active door.
Additionally, if the latch bolt on the inactive door is not aligned with its strike plate when the active door is closed, damage may occur to the locking mechanism.
In order to overcome the above noted problems, U.S. Pat. No. 4,099,753 discloses a locking mechanism that is stated to require less force to operate, and which will not become damaged or cause any damage if the bolt and keeper are not aligned when the doors are closed.
The locking mechanism of U.S. Pat. No. 4,099,753 is nonetheless excessively complicated in that it features a relatively large number of coacting linkages. Moreover, the operative locking movements of the linked parts still require a substantial operating force which does often prevent easy door closing and locking.